Carbide materials are well known in the art of material science. They include a range of compounds composed of carbon and one or more carbide-forming elements such as chromium, hafnium, molybdenum, niobium, rhenium, tantalum, thallium, titanium, tungsten, vanadium, zirconium, and others. Carbides are known for their extreme hardness with high temperature tolerance, properties rendering them well-suited for applications as cutting tools, drilling bits, and similar uses. Multi-element carbides are known for their improved toughness and hardness relative to single element carbides. Single element carbides are typically used with a metal binder to impart toughness.
Multi-carbides are formed by combining two or more carbide-forming elements with carbon. Some multi-carbides have other non-carbide forming elements in the composition, such as nitrogen, but are here referred to simply as multi-carbides since the dominant components are carbide-forming elements. For example, a combination of tungsten and titanium with carbon and nitrogen would be such a multi-carbide material. Some multi-carbide compositions are formed with a deficiency of carbon resulting in some small percentage of carbide-forming element not being converted to a carbide and instead remaining as uncombined elemental metal. These combinations can enhance certain of the favorable qualities of carbides, with some combinations increasing hardness, others increasing toughness, and so forth. Very small variations in composition can greatly affect the material's properties. Many of these variations are well understood by practitioners of the art and are amply published.
Spheres and solid bodies of other specific shapes, whether of carbide or multi-carbide, are difficult to manufacture due to the very properties that make them useful. Their high melting point necessitates a powerful energy source with difficulty in temperature regulation and effect, and their hardness makes them costly to machine.
For example, a primary manufacturing method used to manufacture carbides is to place the elements to be fused on the recessed surface of a large electrode. A very high current is passed from that electrode through the material and into another electrode in proximity, subjecting the material to the heat of an electric arc. This process is effective in fusing the materials, but causes inconsistent mixing of the elements in the compound and some uncontrolled loss of material due to vaporization, phenomena that can greatly compromise the properties of the resulting compound in uncontrolled and unpredictable ways. Hardness is also a challenge, as the manufacturing process results in an irregularly-shaped lump of resulting compound that is generally a few inches in diameter, colorfully known as a “cow chip”. The “chip” is very hard, and is worked into smaller shapes only by percussive shocking or other crushing method that cleaves the chip into useful sizes. These processes leave small cracks in the finished product that greatly reduce both its hardness and its mechanical toughness. Re-melting of the material after crushing imposes high cost, and cannot efficiently achieve regular particle sizes or shapes. Consequently, although carbide is available in small spheres and other preferred shapes, those spheres are not optimally composed, they are irregularly sized, they are expensive, and they are lacking in effectiveness.
The known art currently does not have a process whereby multi-carbide materials can be formed into small and regular shapes without loss of optimized properties due to process variation in manufacture or degradation of material during shaping.
Reducing of particles, also known as comminution, is a very old art, practiced for example by the ancients to produce flour from grain by stone wheel grinding. Later practices required smaller and more regular powders for a variety of industrial applications, and more refined techniques were developed to produce those products, such as media milling. Modem technologies and practices now demand ever-finer particles, measured in microns, thousandths of microns, and even angstroms; and with greater regularity of particle size and purity at these reduced dimensions.
Just as stone wheel grinding could not reliably provide the powders needed for earlier industrial processes, current media mills and similar technologies cannot reliably provide the ultra-fine and ultra-regular particles now required for certain applications.
Various methods for reducing the size of particles have been employed. Many use materials such as spheres, rods or more irregular objects (“grinding media”) to crush or beat the material to be reduced (“product material”) to smaller dimensions by processes known as grinding, milling, comminution, or dispersion. Grinding media range greatly in size, from ore crushers that are several inches in diameter, down to micron-sized particles that are themselves used to mill much smaller particles. Grinding media also vary greatly in shape, including spherical, semi-spherical, oblate spherical, cylindrical, diagonal, rods, and other shapes (hereinafter “shaped media”), and irregular natural shapes such as grains of sand.
Grinding media are used in various devices such ball mills, rod mills, attritor mills, stirred media mills, pebble mills, etc. Regardless of their differences in design, all mills operate by distributing product material around the grinding media and by causing collisions to occur between grinding media units such that product material particles are positioned between the colliding grinding media units. These collisions cause fracturing of product material particles into to smaller dimensions, an effect known descriptively as “size reduction” or “comminution.”
The materials used as grinding media also are frequently used as applied abrasives. For example, such materials are aggregated in molds and held together by a binder such as molten metal that is poured into the mold and cooled, rendering a “hard body” that is impregnated by the binder material. Hard body materials (also known as “hard bodies”) of this kind are used in deep-well drilling and other applications. Similar processes are used to impregnate the materials in grinding discs and wheels. Various adhesives are used to bind the materials to textiles, papers and other strata for use as sandpapers, sanding belts, and similar products.
Different grinding and milling techniques produce different mean product material particle sizes and uniformity. Gross differences in result are obtained primarily as a function of the size and shape of the grinding media. Large grinding media produce relatively large and irregular product material particles that are suitable for coarse processes or for further refinement by finer processes. Small grinding media can be used to produce finer and more regular materials as an end in itself, or to alter crystallite aggregates, or to cause mechanochemical alloying, or some combination thereof. Small grinding media are also used for polishing, burnishing, and deburring. Mills are sometimes used in series, with progressively smaller grinding media employed to further reduce product material particle size in stages. Variation of the shape of the grinding media generally affects the regularity of particle size, the efficiency of the milling process, the total cost to achieve a given size reduction, and other factors. These effects generally are well known in the art.
Extremely small particle sizes are proving to be useful for many new applications. however, the size reduction and regularity necessary for standardized, acceptable results cannot be achieved by any current milling methods. Production now requires alternate particle fabrication methods such as chemical precipitation, either at a fast rate with unacceptable process variation, or at very slow rates, with unacceptable time and expense.
Other important effects are obtained by varying the composition of the grinding media itself. Three material properties dominantly affect grinding media performance: hardness, mass density, and mechanical toughness. Hardness of the grinding medium determines milling effectiveness, mass density determines milling efficiency, while mechanical toughness determines product purity and overall process efficiency. Hard materials transfer energy efficiently in collisions with product material for effective milling, high-density materials increase the energy transfer per collision with product material and thus increase milling efficiency, especially for small-dimension grinding media, and tough materials can be used for longer periods before they fail and contaminate the product material or otherwise require replacement. An ideal milling material is thus very hard, of very high mass density, and very tough. Preferably, those qualities will hold as the size of the grinding media is reduced, and regardless of the chosen shape of the grinding media.
The history of engineering materials for grinding media is a history of accepting tradeoffs among these material properties, as improvement in one of these factors has previously produced an offsetting reduction in one or more of the others. For example, yttria-stabilized zirconia shows good mechanical toughness, but with low mass density. Various metal media have relatively high mass density, but low mechanical toughness. Carbides showed extreme hardness and mass density, even in small dimensions, but with unavoidable media failures that cause unacceptable product contamination and more general process failures that are incompatible with many applications.
U.S. Pat. No. 5,407,564 (Kaliski) is illustrative. Kaliski discloses a range of high mass density, single-element carbides selected from tungsten, thallium, niobium, and vanadium in sizes ranging between 10 and 100 microns with a requirement of high theoretical density. As Kaliski explains, high theoretical density, nonporous materials are needed. These materials showed impressive results in producing fine and regular product material in small quantities under controlled laboratory conditions. Duplication of his example showed his invention to cause contamination of the milled product, as longer-term and higher-volume production attempts failed due to lack of mechanical toughness that caused metallic and other contamination of product material. High density ceramics without metal binders, such as tungsten carbide combined with tungsten di-carbide, also are disclosed by Kaliski as a means to obtain high milling efficiency but with contamination of product material from the grinding media. Kaliski specifically recommends choosing among his claimed materials to select those whose contaminants provide the most good, or at least do the least damage, to the milled product. These materials changed the nature of but did not resolve the product material contaminant issue, and did not solve the mechanical toughness problem. Rather, these materials tended to fail by degradation into hard, fine and irregular shards that acted as abrasives in the media mill, contaminating the product and on one occasion seriously damaging the mill itself.
U.S. Pat. No. 5,704,556 (McLaughlin) discloses ceramic grinding media without metal binders in dimensions of less than 100 microns diameter. While these materials are acceptably hard, and show greater mechanical toughness than those disclosed in Kaliski, they lack adequate density for many applications or for optimum efficiency in others.
The inventor of the present invention made an effort to make suitable grinding media from available spherical carbides, of which only single element carbides are known in the art. Tungsten carbide/tungsten di-carbide spheres were purchased in conformance to Kaliski's specification and used in a shaker mill, but comminution to the degree cited by Kaliski was not evident. Plasma-processed spherical tungsten carbide/tungsten di-carbide was also purchased from another source, in conformance to Kaliski's specification, in sufficient quantity to test on a production scale. This grinding media fractured due to insufficient mechanical toughness, contaminating the product and extensively damaging the media mill. Tungsten carbide failed due to the lack of mechanical toughness despite experimental variation of media velocity, flow rate, material volume, and other milling variables. Grinding media material in conformance to Kaliski's specification was obtained from several difference sources worldwide, but differences in sourcing produced no significant difference in results. In all attempts with all materials supplied to the Kaliski specification, the level of product contamination was a limitation on usefulness.
U.S. Pat. No. 2,581,414 (Hochberg), U.S. Pat. No. 5,478,705 (Czekai), and U.S. Pat. No. 5,518,187 (Bruno) disclose polymer grinding media which show high mechanical toughness and cause relatively benign product material contamination upon grinding media failure. However, they show low hardness and density relative to ceramics. Polymer grinding media thus can be useful in milling relatively soft product materials that are sensitive to product contamination, and in industries that are relatively insensitive to processing cost, such as in drug processing or in dispersing biological cells for analysis, but they are not appropriate for the majority of industrial applications.
U.S. Pat. Nos. 3,690,962, 3,737,289, 3,779,745, and 4,066,451 (all to Rudy) disclose certain multi-carbides for use as cutting tools. Although the multi-carbides disclosed showed a combination of hardness, density and mechanical toughness that promised to be useful for milling, the known geometries for available multi-carbide materials rendered them incompatible with such use. Difficulties included the large size of multi-carbide material that is produced by current manufacturing methods, and difficulty in machining or otherwise manipulating the material into sizes and shapes useful for milling due in part to its hardness and mechanical toughness.
V. N. Eremenko, et al, “Investigations of alloys or the ternary systems W-HfC-C and W-ZrC-C at subsolidus temperatures,” Dokl. Akad. Nauk. Ukr. SSSR, Ser. A No. 1, 83-88 (1976); L. V. Artyukh, et al, “Physicochemical reactions of tungsten carbide with hafnium carbide,” Izv. Akad. Nauk SSSR, Neorg. Mater., No. 4, 634-637 (1976); and T. Ya. Velikanova, et al, “Effect of alloying on the structure and properties of cast WC1-x Materials,” Poroshkovaya Metallurgiya, No. 2 (218), 53-58, (1981) teach how sensitive the properties of single element carbides can be to small additions of other carbide forming elements. This fact has greatly inhibited research into multi-carbide elements.
As summarized above, the grinding media of the prior art all suffer some technical disadvantage resulting in a proliferation of grinding media materials creating a significant economic burden and also resulting in technically inferior milled products due to contamination.